Spine disturbances are a common feature of intellectual developmental disorders (IDD) such as Down Syndrome (DS), which occurs in ~ 1 in 700 live births and is associated with an average IQ of 50. In DS, cortical dendritic spines are often described as appearing immature (long, thin and tortuous) and spine densities are reduced. The cellular mechanisms underlying these morphological disturbances are not known but likely involve dysregulation of the spine actin cytoskeleton that largely determines spine shape and is critical for plasticity underlying memory encoding. Our previous work has shown that function of one Rho-family GTPase pathway involved in regulating spine filamentous (F-) actin, the Rac cascade, is markedly disturbed in a mouse model of another IDD, Fragile X syndrome; this aligns with observations that several IDDs exhibit abnormalities in Rho GTPase pathways. However, it is not known the degree to which specific defects in spine Rho GTPase signaling are shared across IDDs or converge on the same down-stream proteins that directly regulate spine F-actin. In preliminary studies using fluorescence deconvolution microscopy, we evaluated two Rac pathway proteins, p21-activated kinase 3 (PAK3) and Arp2, in middle-aged human DS parietal cortex. The results demonstrate that levels of both proteins are reduced at excitatory, PSD95-immunopositive (+) synapses suggesting that the actin regulatory machinery is indeed disturbed in DS. The results also suggest the possibility that, like Fragile X syndrome, DS exhibits abnormalities specific to the Rac cascade that regulates the branching and stabilization of the spine actin network. The proposed studies will expand upon these findings and test the hypothesis that trisomy giving rise to DS leads to disturbances in the dendritic spine Rac GTPase cascade while leaving elements in the RhoA cascade relatively normal. Aim 1 studies will further test if abnormalities in actin regulatory proteins are present in DS individuas across a broad age range or preferentially at later ages, and if these perturbations are greatest in DS individuals with Alzheimer's Disease (AD) tau pathology. Findings will provide insight as to whether the actin signaling disturbances are core features of DS, or secondary to emergent AD pathology. Aim 2 will then test if abnormalities in Rac pathway proteins are present in the Ts65Dn mouse model of DS that exhibits both spine and synaptic plasticity abnormalities. Confirmation of this point is essential for the use of the Ts65Dn model in preclinical studies aimed at devising therapies to offset spine defects, and facilitate learning, in DS. Pertinent to this, we have shown that manipulation of signaling through several synaptic modulatory receptors can dramatically alter local actin regulatory signaling and, in some cases, restore normal actin remodeling, synaptic plasticity, and cognitive function in otherwise impaired animals. Thus, the proposed studies will determine if actin regulatory deficits are present in DS spines that might be similarly responsive to actin based strategies for cognitive enhancement.